CITRIC ACID. 113 



to dryness, the mass being stirred from time to time to ensure 

 thorough incorporation. To the mixture is now added 1 c.c. of 

 a solution containing 25 c.c. hydrochloric acid and 10 grammes 

 of oxalic acid in 100 c.c. of water, and the mass is again dried. 

 The same operations are carried out with 10 c.c. of a standard 

 solution of boric acid [1 c.c. being equal to 0-1 milligramme of 

 boron trioxide (B 2 3 )]. 



The colour having been obtained in both cases, the sand 

 is extracted with ordinary alcohol. 



The coloured solution obtained from the milk is diluted with 

 alcohol or methylated spirit until the colour is of the same degree 

 of intensity as that formed from the standard ; and the amount 

 of boric acid is arrived at by an obvious calculation. 



The colours are compared in two tubes of the same internal 

 sectional diameter (about 1 centimetre) placed vertically against 

 a white porcelain plate. 



On comparing the two solutions it will be found occasionally 

 that a certain amount of orange tint is exhibited by one or the 

 other, due to the presence of curcumin in slight excess. When 

 this is observed the tints must be made the same by the cautious 

 addition of solution of curcumin to the solution which does not 

 show the orange tint. 



The result of a number of experiments made with known 

 amounts of boric acid and borates show that the process is 

 reliable and accurate. 



Estimation of Citric Acid in Milk. The proteins are precipi- 

 tated by acid mercuric nitrate (p. 155) and a measured volume 

 of the clear filtrate neutralised exactly with dilute caustic soda 

 solution, using phenol-phthalein as indicator. A white precipi- 

 tate of mercury and calcium phosphate and citrate is thrown 

 down, collected on a filter, and washed with water ; it is removed 

 from the filter, suspended in water, and a little dilute hydro- 

 chloric acid added ; sulphuretted hydrogen is passed through to 

 precipitate the mercury as mercuric sulphide. After filtration, the 

 solution is boiled to remove sulphuretted hydrogen, and, after 

 the addition of a little calcium chloride, cooled. It is then neutral- 

 ised carefully, phenol-phthalein being used as indicator ; the 

 precipitate of calcium phosphate is filtered off, and the solution 

 boiled and concentrated to a small bulk ; the calcium citrate is 

 thus precipitated. This should be washed with boiling water, 

 collected on a small filter and ignited. To the ignited residue 

 an excess of standard hydrochloric acid is added and the excess 

 titrated back with standard alkali, methyl orange being the best 



N 

 indicator. Each cubic centimetre of ^. hydrochloric acid used 



represents 0-0064 gramme of citric acid 



